The present invention relates to integrated circuits, and more particularly, to a programmable integrated bandgap operative at relatively low voltages.
Bandgap reference voltage generators (alternatively referred to as bandgap reference circuits) are used in a wide variety of electronic circuits, such as wireless communications devices, memory devices, voltage regulators, etc. A bandgap reference circuit often supplies an output voltage that is relatively immune to changes in input voltage or temperature.
A bandgap reference circuit is typically adapted to use the temperature coefficients associated with physical properties of the semiconductor devices disposed therein to generate a nearly temperature-independent reference voltage. A bandgap reference circuit operates on the principle of compensating the negative temperature coefficient of VBE—which is the base-emitter voltage of a bipolar transistor—with the positive temperature coefficient of the thermal voltage VT. In its most basic form, the VBE voltage is added to a scaled VT voltage using a temperature-independent scale factor K to supply the reference voltage Vref, as shown below:Vref=VBE+K*VT  (1)
Because voltage signals VBE and VT exhibit opposite-polarity temperature drifts, parameter K may be selected such that voltage Vref is nearly independent. As is known to those skilled in the art, thermal voltage VT is equal to kT/q, where, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electron charge.
In addition to being temperature independent, a bandgap reference circuit is ideally also adapted to supply a substantially stable and unchanging output reference voltage despite variations in the input voltage levels received by or the capacitive loading applied to the bandgap circuit. Accordingly, an ideal bandgap reference circuit output is also immune to ripples or noise that is typically present in the power source supplying voltage to the bandgap reference circuit. However, most bandgap reference circuits exhibit non-ideal characteristics. One measure of the ability of a bandgap reference circuit to suppress or reject such supply ripple or noise voltages is referred to as the power supply ripple rejection (PSRR).
The growth in demand for battery-operated portable electronic devices, such as wireless communications devices and personal digital assistance devices, has brought to the fore the need to develop low voltage, low power systems. For instance, many portable wireless systems are being designed to operate using batteries that supply, for example, 1.2 volts. Designing a bandgap reference circuit adapted to operate at such low voltages poses a challenging task.
There continues to be a need for a bandgap reference circuit that is operable at low voltages and is further adaptable to achieve different output voltages having nearly zero temperature coefficients.